1. Field of the Invention
The present invention relates to a method of fabricating a barrier layer and, more particularly, to a method of fabricating a barrier layer on a top surface of metal in damascene structures utilizing ion implantation.
2. State of the Art
One of the main problems confronting the semiconductor processing industry, in the ultra large scale integration (“ULSI”) age, is that of capacitive-resistance loss in wiring levels. This has led to a large effort to reduce the resistance of, and lower the capacitive loading on, the wiring levels. Since its beginning, the industry has relied on aluminum and aluminum alloys in metallization layers. To improve conductivity in the wiring, it has been proposed that copper metallurgy be substituted for the aluminum metallurgy. However, several problems have been encountered in the development of the copper metallurgy. One of the main problems is the fast diffusion of copper through insulative materials, such as silicon and silicon dioxide (“SiO2”), to form an undesired copper oxide compound. In addition, copper is known to cause junction poisoning effects. These problems have led to the development of a liner to separate the copper metallurgy used in the metallization layer from the insulative material. However, copper does not adhere well to oxygen-containing dielectric materials or to itself. Therefore, the liner functions as both an adhesion layer and a barrier layer. In other words, the liner is used to provide adhesion between the copper metallurgy and the insulative material and also to prevent the diffusion of copper through the insulative material.
Liner materials that act as a barrier layer to prevent the diffusion of copper have been investigated by numerous researchers. The use of titanium (“Ti”), zirconium (“Zr”), or hafnium (“Hf”) in the barrier layer has been disclosed. Anonymous, “Improved Metallurgy for Wiring Very Large Scale Integrated Circuits,” International Technology Disclosures, v. 4 no. 9, (Sep. 25, 1986). Chemical vapor deposition (“CVD”) of titanium nitride (“TiN”) has also been proposed. C. Marcadal et al., “OMCVD Copper Process for Dual Damascene Metallization,” VMIC Conference Proceedings, p. 93–98 (1997). It is currently believed that an optimal liner material for a barrier layer is either a metal, such as tantalum (“Ta”) or tungsten (“W”), or a compound such as tantalum nitride (“TaN”) or trisilicon tetranitride (“Si3N4”). Changsup Ryu et al., “Barriers for Copper Interconnections,” Solid State Technology, p. 53–56 (1999).
Researchers have also proposed an alternate method of forming the barrier layer where the copper of the metallization layer is alloyed with a reactive element, such as aluminum or magnesium. S. P. Muraka et al., “Copper Interconnection Schemes: Elimination of the Need of Diffusion Barrier/Adhesion Promoter by the Use of Corrosion Resistant, Low Resistivity Doped Copper,” SPIE, v. 2335, p. 80–90 (1994) (hereinafter “Muraka”); Tarek Suwwan de Felipe et al., “Electrical Stability and Microstructural Evolution in Thin Films of High Conductivity Copper Alloys,” Proceedings of the 1999 International Interconnect Technology Conference, p. 293–295 (1999). Copper alloys with 0.5 atomic percent aluminum or 2 atomic percent magnesium were used. The reactive element reacted with SiO2 to form dialuminum trioxide (“Al2O3”) or magnesium oxide (“MgO”), which acted as a barrier to the further diffusion of the copper into the SiO2.
Similarly, in U.S. Pat. No. 5,130,274 issued to Harper et al. (hereinafter “Harper”), an oxide layer that acts as a barrier is disclosed. To form the barrier, a copper alloy containing an alloying element, such as aluminum or chromium, is deposited as a layer. The alloying element reacts with SiO2 or a polyimide to form an oxide that functions as a barrier compound.
Semiconductor products that incorporate some of these solutions to the problem of copper diffusion have begun to ship, on a limited basis. However, a problem of how to achieve the lowest possible resistivity in ever smaller lines still remains. As shown in Panos C. Andricacos, “Copper On-Chip Interconnections,” The Electrochemical Society Interface, p. 32–37 (Spring 1999) (hereinafter “Andricacos”), the effective resistivity obtainable by use of barrier layers was approximately 2 μΩ-cm with a line width greater than 0.3 μm. The effective resistivity undesirably increases for narrower lines. The alloys investigated by Muraka had similar resistivity values to those found by Andricacos. Muraka also found that the use of 0.5 atomic percent aluminum in the copper was apparently insufficient to provide complete protection from copper diffusion into the SiO2. However, a significant reduction in the rate of copper penetration through the SiO2 was achieved. The maximum solubility of aluminum in copper is 9.4 weight percent or approximately 18 atomic percent and the maximum solubility of magnesium in copper is 0.61 weight percent or approximately 0.3 atomic percent. Thus, the alloys investigated in Muraka were saturated with magnesium but were far below the saturation limit when aluminum was used as the reactive element.
Other researchers have investigated the capacitive loading effect with various polymers, such as fluorinated polyimides, to determine if the polymers are possible substitutions for SiO2 as an insulative material. Several of these polymers have dielectric constants that are considerably lower than the dielectric constant of SiO2. However, as in the case with SiO2, the polymers also exhibit incompatibility problems with copper metallurgy. It has been shown that polyimide, and many other polymers, react with copper during the curing process to form CuO2, which is dispersed within the polymer. D. J. Godbey, L. J. Buckley, A. P. Purdy and A. W. Snow, “Copper Diffusion in Organic Polymer Resists and Inter-level Dielectrics,” at the International Conference on Metallurgical Coatings and Thin Films, San Diego, Calif., Apr. 21–25, 1997, Abstract H2.04 p. 313 (hereinafter “Godbey”). CuO2 is conductive, so its presence raises the effective dielectric constant of the polymer and, in many cases, also increases the conductivity of the polymer.
Andricacos notes that the use of a copper conductor along with the barrier layer provides a significant improvement in conductivity over the Ti/AlCu/Ti sandwich structure currently used in the industry. Andricacos also noted that as the line width decreases, even a thin barrier layer has a significant effect on the resistance of the composite line. The solutions proposed by Harper and Muraka attempted to address this problem by forming the barrier layer in situ by chemically reacting the insulative material and the copper alloy. The barrier layer is formed in the area that was previously the insulative material, leaving the conductor width and height unaffected. However, these processes also affect the resistivity of the conductor because the use of an aluminum alloy, even at a concentration so low as not to be completely effective, shows a measurable increase in resistance compared to that of an unalloyed copper line. While the process of Harper uses only one layer of the copper alloy, the one layer has a significantly high concentration of aluminum and, therefore, the final structure will have an increased resistivity.
As minimum dimensions shrink, the use of even a 20 Angstrom (“Å”) layer of an alloy having a high resistivity will significantly affect the total resistivity of the conductor composite. For example, a 200 Å layer on both sides of a 0.1 μm trench is 40 percent of the total trench width. Therefore, at the same time that the dimensions of the conductor element decrease, the specific resistivity increases, which provides a high resistivity at the very time a conductor having a low resistivity is desired.
It has also been shown that there is a significant difference in the amount of copper oxide formed when a polyimide is used as the insulative material if the acidity of the polymer solution is low. The acidity of the polymer solution is typically low if the precursor used in the formation of the polyimide is an ester instead of an acid. As shown in Godbey, when PI-2701 is used, the amount of oxide formed is reduced by a factor of approximately four in comparison to films that use a similar chemistry but that are prepared from an acid precursor. PI-2701 is a photosensitive polyimide from E.I. du Pont de Nemours & Company (Wilmington, Del.) that starts from an ester precursor. It is thought that the slight acidity of PI-2701 comes from the photo-pac or the process used to form it. The films produced in Godbey were prepared by curing the liquid precursor in an air environment or a near, but not completely, inert environment. It is well known that the copper oxide will not form in, and is reduced by, a high purity hydrogen atmosphere.
Muraka discloses that the use of Ti as a barrier layer was found to increase the resistivity of the copper film significantly when heat-treated at temperatures of 350° C. or greater. If the heat treatment was carried out in hydrogen, no increase in resistivity was found. As this temperature is above the eutectoid temperature of a Ti-hydrogen system, the formation of TiH is assumed to have occurred. Muraka also indicates that a similar increase in resistivity is seen with Zr- and Hf-containing copper alloys. However, no data in support of this assertion is provided.
Other researchers have disputed Muraka's conclusions. Saarivirta, M. J., “High Conductivity Copper Rich Cu—Zr Alloys,” Transactions of the Metallurgical Society of AIME, 218, p. 431–437, (1960) (hereinafter “Saarivirta”); U.S. Pat. No. 2,842,438 to Saarivirta et al. Equilibrium phase diagrams of Cu—Ti and Cu—Zr systems show that the solubility of Zr in copper is more than ten times less than that of Ti. Metals Handbook, v. 8, p. 300-2 (8th Ed.). It should also be noted that a series of Cu—Zr alloys have been disclosed that have good electrical conductivity.
Saarivirta shows that alloys containing more than about 0.01 weight percent of Zr have a significant loss of conductivity in an as-cast state. It has also been shown that the conductivity of a 0.23 weight percent Zr alloy is restored to above 90 percent of the international annealed copper standard for conductivity (“IACS”) when the alloy, in the cold drawn state, is heat-treated above 500° C. for one hour. This indicates that a significant amount of the Zr, which was in solid solution in the as-cast state, has precipitated as pentacopper zirconium (“Cu5Zr”). Therefore, it can be seen that if the Zr content in the copper is kept low, the conductivity of the resulting metallurgy is above 95 percent of IACS. If a Zr layer is deposited on top of a copper layer, the temperature of deposition of the Zr should be kept below 450° C., with a temperature of 250° C.–350° C. being preferable. This deposition typically occurs in a single damascene process or at the bottom of vias in a dual-damascene process. When the deposition temperature is kept in this range, a thin layer of Cu5Zr tends to form initially, thus inhibiting the diffusion of Zr into the copper. Even at 450° C., the solubility is low enough to provide very good conductivity. Although Zr and Ti have many similar properties, their solubility in copper differs by more than a factor of 10. Therefore, the use of Zr is preferred over Ti for this application.
Methods of forming barrier layers and seed layers by ion implantation are disclosed in U.S. Pat. Nos. 6,420,262 and 6,376,370, both to Farrar, incorporated in their entirety by reference herein. In U.S. Pat. No. 6,420,262, a transition metal, a representative metal, or a metalloid is deposited on an insulator using an ion implantation technique. An inhibiting layer or barrier layer is formed by a reaction between the transition metal, the representative metal, or the metalloid and the insulator. The barrier layer prevents copper from a metallization layer from diffusing into the insulator. In U.S. Pat. No. 6,376,370 a barrier layer and a seed layer are deposited using ion implantation. The barrier layer is formed by depositing Zr, Ti, or Hf by a low energy ion implantation. The seed layer is formed by depositing aluminum, copper, silver, or gold by a low energy ion implantation. Copper, gold, and silver have lower resistivities than aluminum and also have a significantly lower adhesion to oxides than aluminum. However, all of these materials have relatively poor adhesion to polymers.
The processes described above offer significant improvements to the barrier adhesion art for bottom and sidewalls of trenches of a damascene or dual damascene structure. While techniques for forming barrier layers that separate copper from the trenches in a damascene and/or dual damascene structure are known, few acceptable techniques exist for forming the barrier layer on a top surface of a metal in a damascene structure. The few processes that exist require considerable processing and process complexity. For instance, in U.S. Pat. No. 6,426,289 issued to Farrar, a thin layer of zirconium is deposited on a top surface of a copper metal layer to form a barrier layer. The zirconium is implanted into the copper using a low energy implantation technique. However, the process uses a sacrificial, multilayer dielectric structure and selective etching to form the barrier layer and, therefore, requires modifications to the damascene process.
It would be desirable to provide a barrier layer for the top surface of a semiconductor device structure that does not require complex processing or modifications to current processes. To reduce processing complexity, it is desirable to form the barrier layer without using a mask to define the barrier layer.